Triaxial creep behavior of oil shale was investigated in the laboratory at simulated in-situ conditions. A range of temperature and stress conditions were chosen to represent those within the inter-chamber pillars of a modified in-situ retorting operation. Oil shale in grades up to 60 gallons per ton have been tested. Experimental data and a constitutive model have been presented here.
Creep is a significant aspect of the mechanical behavior of oil shale. This has been confirmed by actual measurements on pillars in experimental underground oil shale mines (Agapito, 1972), and laboratory tests on oil shale samples (Miller, et al., 1979; Chu and Chang, 1980). It is, therefore, implied that time-dependent deformational behavior of oil shale will be a governing factor in all oil shale mine designs. In an in-situ, or modified in-situ retorting operation for recovering oil from oil shale, time dependent deformations are more pronounced due to elevated temperatures. In a modified in-situ retorting operation, part of the oil shale initially is conventionally mined and the retort chambers are prepared by rubblization of shale in place. Creep behavior of oil shale in this case may significantly affect the stability of the interchamber pillars and ground subsidence as well as the functioning of the retorts themselves due to resulting permeability loss. This paper describes a systematic investigation of the creep behavior of oil shale under temperature and stress conditions expected within an inter-chamber pillar. The investigation consists of performing a series of triaxial creep tests and constitutive modelling. Interest in the mechanical properties and behavior of oil shale is quite recent. Sellars, et al. (1972) and Zambas, et al. (1972) seem to be the first to report a limited systematic investigation of time-dependent mechanical behavior in oil shale. Maximum duration of tests was 1,000 hours. The main findings of these tests were that practically no creep occurred below a critical nominal stress of 13.79 MPa; very low initial creep strain rates occurred and rapidly reduced to zero (in only a few days) at stress levels between 13.79 MPa and 41.37 MPa (a logarithmic creep strain versus time behavior was observed during this period); creep rupture occurred in samples with oil content above 30 gallons per ton (GPT) at stress levels of 55.16 MPa (corresponding to approximately 80% of compressive strength). Agapito and Page (1975) have described the long term vertical deformation behavior of actual oil shale pillars in the Colony Oil Shale Mine. They observed that the creep strain rate measured in a pillar was a function of the vertical pillar stress and proposed a linear relationship. Chong, et al. (1978) performed creep and relaxation tests on oil shale samples from the Wyoming Green River formation subjected to uniaxial compression at room temperature. Samples with oil content from 10 GPT to 50 GPT were tested at stress levels of 25% to 75% of their ultimate compressive strength values and for times up to 80 hours. A standard linear visco-elastic model has had some success in describing the behavior.